Pusher cowling ideas

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Eugene

Well-Known Member
If it were mine, I would avoid pulling the flow in and then running it nearly straight, then tapering it again. Since we can not avoid some sort of taper to a point or nearly to a point, I fill in the reversed curve there at the aft end of the cowling, and then do the pressure recovery spinner.
All I really understood here is "pressure recovery spinner". Sorry, I need translation for rest of it.

wsimpso1

Super Moderator
Staff member
Log Member
And looks like we will have low pressure everywhere on top and everywhere on the bottom. Because velocity will be higher and as a result pressure will be low.

View attachment 115449

So, we said that we can use this shape as backside of fuselage or spinner for pusher propeller. And pressure will be low?
Look at TOWS Appendix I again. v/V is local velocity divided by free stream velocity. v/V is velocity ratio. V/v less than 1, V is high, pressure is low. V/v is less than one, pressure is higher, and it is a continum. You actually get a little push forward at the aft end. You want that.

If this guys decided to make fat tail boom, it will become low pressure tail? But because they decided to make it skinny, it is high-pressure tail?

View attachment 115451
The thicker aft fuselage still gets the push from pressure recovery, but it also has a lot more wetted area, which is all drag. Better to taper it down nicely and reduced drag by having less wetted area.

If I made my tail boom like a picture of the law, it should become high-pressure tail boom. And airflow will stay attached. But airflow can stay attached on high-pressure or low-pressure areas. So, what difference does it make?

View attachment 115453
Here is that picture of what the Skyboy planview again. Understand something here. That is just what the widest part of the fuselage looks like when viewed from above. Your fuselage is not a solid of rotation of that shape. Does the wing and tail block off flow? Nope, the wing and tail allow flow by them pretty easily. So does the flaired boom you had. To get some understanding of what the fuselage looks like to the air, you need the cross section at fuselage stations as you go aft. The aft flare you show is just through the landing gear legs. Draw cross sections, figure out what the cross section area is, and that is about how the air sees it. Increasing section, v/V is higher, p is lower. Decreasing section, v/V is lower, p is higher.

Now if this does not "click" for you, so what? Accept that it does this and move on.

Billski

wsimpso1

Super Moderator
Staff member
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Here is a real life airflow question. I don’t know if you can see this packet that I show in red on the picture, but seems to me that there will be flow direction change. My guess that I will need to fill this packet so air is flowing smoothly.

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I really do not understand the bumps and cheeks. If you can, ideal is a smooth curve from front to back around the stuff. The chines and ridges and such are just more corners for the flow to run over... I do like most of the developed shape and the fillets. The more you can make the walls vertical and section constant in the back half of the wing chord, the less fillet you need. The more you pull in the lower part of the cowling, the more you need those fillets. That skin area is going to be there one way or the other. Even with the chines and ridges, it does look pretty good.

Billski

wsimpso1

Super Moderator
Staff member
Log Member
All I really understood here is "pressure recovery spinner". Sorry, I need translation for rest of it.
I can not say it much plainer... Look at the picture in post 286. As you go aft along the line drawn in, the line draws in, then straightens out, then transitions to the spinner and draws in again. That is curved once, twice, three times. Ideally, we curve in, than reflex out to the smallest size, and that is it. Every time we change the direction of the air, it costs us something, so lets do as little of it as possible.

Eugene

Well-Known Member
I can not say it much plainer... Look at the picture in post 286.
It's not you, it's my English. Plus I didn't know what picture to look at. I got it this time. Thank you! We want to curve one only time if possible. There is a price to pay for any kind of zigzagging. That is why I ask this morning about airflow direction change. I will work on it.

WonderousMountain

Well-Known Member
So, having demonstrated I can't free-hand,
I submit the non sinusoidal Nacelle finish.

Okay, also original GOE airfoil for comparison.

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Eugene

Well-Known Member
So, having demonstrated I can't free-hand,
I submit the non sinusoidal Nacelle finish.
View attachment 115502
Okay, also original GOE airfoil for comparison.
View attachment 115503
I think I got it. This is about how my spinner is going to look like with 14 inch diameter and 22-24 inches long.

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WINGITIS

Well-Known Member
Another pusher for info:

WINGITIS

Well-Known Member
Have a look through this study, it covers just about everything including the effect of the blades being there and creating offset airstreams....

Its the best document I have seen.....its from 2020

ITS TOO LARGE TO LOAD HERE...

Cheers
K

WINGITIS

Well-Known Member
A modern variant....

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WINGITIS

Well-Known Member
Eugene I think this is where you need to go to get the total shape correct before the spinner is finally decided.

I think it is what you were trying to ask us all along??

You wanted an ACTUAL airfoil shape for the sides of your fairing.

As an example I choose an Eppler 556 image from Airfoil Tools and used MS PAINT to reverse it and split it apart and then apply it to your max width and spinner width dimensions in the attached example.

You can see that the Eppler 556, a wind turbine airfoil, has its maximum thickness too far forward so you would have to find a higher speed laminar flow type airfoil to get it fatter in the middle before you start the MS PAINT process, but you should be able to do that fairly easily from the many Airfoil Tools images.

Once you have found one that fits closely over the shape you have you will then be in a better position to determine the final spinner.

AFTER THAT, from you Cruise and Max speeds, Cruise Thrust and Max Thrust and Prop diameter( I dont know what they are) those two speed Reynolds numbers can be calculated, because the speed over the cowling is different with engine off to say full thrust, independent of the aircraft speed.

One could then optimize the airfoil chosen for those two Reynolds Numbers...IF ONE DESIRED.....

That would then effectively choose the spinner length for you....

BUT MORE LIKELY, once you find the correct airfoil for your fairing sides it will become obvious what the spinner should look like without those calculations...

Hope that helps,

Cheers
K

WINGITIS

Well-Known Member
For instance this could be a good candidate, using the lower half! It is pretty close....

You can even see that the 1:14 ratio Ogive is a little too long as that Thesis paper and many of the other examples in production would indicate.

Or the Ogive is a little too bulbous and you could reduce that setting in the spreadsheet. Change Cell J9 from a "0" to "1.5" for example.

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WINGITIS

Well-Known Member
Here is that J9=1.5 versions as well.

A bit slimmer, could maybe do with even more...

Attachments

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Well-Known Member
Here is that J9=1.5 versions as well.

A bit slimmer, could maybe do with even more...

View attachment 115529
You've still got an s-bend in the airflow, although that's right where the prop is interfering with it, and I have no idea how that will affect things.

If you're basing the shape on a partial aerofoil, why not start with a symmetrical one?

WINGITIS

Well-Known Member
You've still got an s-bend in the airflow, although that's right where the prop is interfering with it, and I have no idea how that will affect things.

If you're basing the shape on a partial aerofoil, why not start with a symmetrical one?
I have posted the spreadsheet so people can make whatever prop spinner shape they want.

Some people like it to be flat where the blades are, some dont etc etc.

I was just demonstrating to Eugene the technique to do the job, I am not doing it for him because he has his own requirements.

Because I am selecting ones more suited for low performance aircraft, from HAWT wind turbine blades. Again I am showing him and others the techniques.

There are thousands of airfoils to go through if someone wants to do that.

Do you have any in mind that are widest in the middle, which Eugene wants?

Someone else might be able to grab what they think is the ideal one from Airfoil Tools and use MS Paint to show Eugene and the rest of us what they think is good.

The HAWT ones are also more up to date..

If you study that THESIS paper you will see there are many factors involved.

As the MQ-9 Spinners show and the thesis tests trend towards, the direction is staying towards shorter ones.....

There is no dream ideal for the configuration Eugene has, because of the large frontal area of the craft, limited CDA and all the interference issues and sub-optimal configuration items.

He is endeavoring to make a biggish difference in the performance of his craft using the simplest methods.

Otherwise he would be doing it in CAD with CFD.

Here is a more pointed Ogive...its easy to change.

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Eugene

Well-Known Member
Have a look through this study, it covers just about everything including the effect of the blades being there and creating offset airstreams....

Its the best document I have seen.....its from 2020

ITS TOO LARGE TO LOAD HERE...

Cheers
K
This attached document is very interesting but much more complex than I can ever understand with my English proficiency. I spent some time trying to find general conclusions in plain English, but didn’t see any that I can translate for myself. Sorry.

WINGITIS

Well-Known Member
Hi Eugene

The broad outcome of the paper, as I see it, is that the propeller blades being present change the flow to be assymmetric and so the spinner does have an effect related to that rotational flow, and in a different way to just as if it was an airfoil with no blades, which seems to indicate that the shorter ones are better.

One or two of the short ones with a bit of an extension as you have were also ok, but they do not seem to be prevalent on production aircraft, the Defiant and the MQ-9 are aircraft that have no budget limitations in terms of spinner costs and design, the Defiant is traditional(AT THIS STAGE) and the MQ-9 is relatively short for its diameter and is similar to some in the study.

Some of the aspects of the MQ-9 spinner are related to RCS reduction and heat/IR management for the Turboprop, but as the study shows they are also ok for flow.

On the MQ-9 it looks like heat would come out of the blade holes in the spinner, but that is not the case, there is a blanking plate to prevent that. So the large holes may even be to let cooling air into the spinner to reduce the IR signature.

Because some of the spinners appear to have got FAIRLY HOT from the exhaust flowing over them, because there is some discoloration evident.

Underneath the covers of the MQ-9 there are all sorts of ducts managing heat/airflow and its difficult to judge how much is radiated to the fuselage SKIN and how much is recombined with the exhaust and ejected out of the top rear. Thats a tricky science all by itself....

Yours is coming out the bottom rear, so in that aspect is similar in terms of the spinner seeing the vertical temperature variation.

BUT YOUR NOT BEING TARGETED FROM BELOW, BY MISSILE CONTROL SYSTEMS, WHILST AT 28,000 FEET, so I think you are OK there

Cheers
K

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wsimpso1

Super Moderator
Staff member
Log Member
I have already Indicated that ideally the cowling should be a vertical walled straight section through the wing. This is particularly important in plan view. In profile view, this is less important.

Being as we can not do that forward, and it is most important to do it aft, then I would aim for a nice smoothly increasing section from inlet to the widest place, carry that section as far aft as the wing trailing edge, and then taper to the end of the spinner. I would do this in plan view, and then aim for the smoothest curve in profile view.

If I just could not trust myself on a “nice smooth” shape, I would be inclined to select a symmetric laminar foil, scale the chord to run from forward of the inlet to the aft end of the spinner and scale the thickness to something like the width. Accept that the air will pile up on the inlet and sort of look like the front 10-15% of chord. Also accept that the walls will be straight for a ways. Maybe you cut a smaller scaled foil in two at the fat part, then put in a straight section in the middle. And if you have a foil with a cusp on it, the spinner then becomes a pressure recovery shape.

In profile view, use the same basic foil with whatever ”stretch” from plan view, and scale the thickness to fit.

These would only guides to sort of keep nice shapes, but if one needs guides, there you go.

Billski